EN 12255-6:2023

SLOVENSKI STANDARD
01-september-2023
Nadomešča:
SIST EN 12255-6:2002
Čistilne naprave za odpadno vodo - 6. del: Postopek z aktivnim blatom
Wastewater treatment plants - Part 6: Activated sludge process
Kläranlagen - Teil 6: Belebungsverfahren
Stations d'épuration - Partie 6: Procédé à boues activées
Ta slovenski standard je istoveten z: EN 12255-6:2023
ICS:
13.060.30 Odpadna voda Sewage water
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN 12255-6
EUROPEAN STANDARD
NORME EUROPÉENNE
July 2023
EUROPÄISCHE NORM
ICS 13.060.30 Supersedes EN 12255-6:2002
English Version
Wastewater treatment plants - Part 6: Activated sludge
process
Stations d'épuration - Partie 6: Procédé à boues Kläranlagen - Teil 6: Belebungsverfahren
activées
This European Standard was approved by CEN on 28 May 2023.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2023 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN 12255-6:2023 E
worldwide for CEN national Members.

Contents
Page
European foreword . 4
Introduction . 6
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 7
4 Symbols and abbreviations . 8
4.1 Symbols . 8
4.2 Indices (not included in the symbols or abbreviations below). 10
4.3 Abbreviations . 11
5 Requirements . 12
5.1 General. 12
5.2 Planning . 13
5.2.1 Basic information. 13
5.2.2 System selection . 14
5.2.3 Biological reactors . 18
5.2.4 Clarifiers . 18
5.2.5 Environmental impact . 19
5.3 Detailed Design . 19
5.3.1 Flow-splitting . 19
5.3.2 Biological reactors . 19
5.3.3 Mixing . 20
5.3.4 Aeration . 22
5.3.5 Secondary clarifiers . 26
5.3.6 Return and surplus sludge systems . 28
5.3.7 Internal recirculation . 28
5.3.8 Control and automation . 28
6 Test methods . 30
Annex A (informative) Design of biological reactors . 31
Annex B (informative) Raw wastewater characteristics . 32
Annex C (informative) Removal efficiency of primary clarifiers . 33
Annex D (informative) External carbon sources . 34
Annex E (informative) Sludge age (MSRT) and aerobic sludge age (MASRT) . 35
Annex F (informative) Surplus sludge production . 37
Annex G (informative) Denitrification capacity . 39
Annex H (informative) Oxygen Consumption . 40
Annex I (informative) Iterative calculation of the volumetric ratio of denitrification reactors
(V /V ) . 42
Den R
) . 43
Annex J (informative) Reactor volume (VR
Annex K (informative) Internal recirculation ratio (IRR) . 44
Annex L (informative) Alkalinity . 45
Annex M (informative) Aerobic selectors . 47
Annex N (informative) Design based on F/M-ratio . 48
Annex O (informative) Sludge volume index (SVI) . 49
Annex P (informative) Solids concentration of the return sludge (C ) . 50
TSS,RS
Annex Q (informative) Return sludge flow (Q ) and total suspended solids concentration in
RS
the biological reactor (C ) . 51
TSS,R
Annex R (informative) Surface area (A ) of final clarifiers . 53
Cla
Annex S (informative) Depth (hCla) of final clarifiers . 54
Annex T (informative) Scraper Design. 55
Annex U (informative) Return sludge balance . 57
Annex V (informative) Influent structures . 58
Annex W (informative) Design of a fine bubble aeration system . 59
Bibliography . 62
European foreword
This document (EN 12255-6:2023) has been prepared by Technical Committee CEN/TC 165 “Waste
water Engineering”, the secretariat of which is held by DIN.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by January 2024, and conflicting national standards shall
be withdrawn at the latest by January 2024.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
This document supersedes EN 12255-6:2002.
This is the sixth part prepared by Working Group CEN/TC 165/WG 40, relating to the general
requirements and processes for treatment plants for a total number of inhabitants and population
equivalents (PT) over 50.
The EN 12255 series with the generic title “Wastewater treatment plants” consists of the following Parts:
• Part 1: General construction principles
• Part 2: Storm management systems
• Part 3: Preliminary treatment
• Part 4: Primary treatment
• Part 5: Lagooning processes
• Part 6: Activated sludge process
• Part 7: Biological fixed-film reactors
• Part 8: Sludge treatment and storage
• Part 9: Odour control and ventilation
• Part 10: Safety principles
• Part 11: General data required
• Part 12: Control and automation
• Part 13: Chemical treatment — Treatment of wastewater by precipitation/flocculation
• Part 14: Disinfection
• Part 15: Measurement of the oxygen transfer in clean water in aeration tanks of activated sludge plants
• Part 16: Physical (mechanical) filtration
NOTE Part 2 is under preparation.
NOTE For requirements on pumping installations at wastewater treatment plants see EN 752, Drain and sewer
systems outside buildings — Sewer system management and EN 16932 (all parts), Drain and sewer systems outside
buildings — Pumping systems.
Any feedback and questions on this document should be directed to the users’ national standards body.
A complete listing of these bodies can be found on the CEN website.
According to the CEN-CENELEC Internal Regulations, the national standards organisations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria, Croatia,
Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland,
Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of North
Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and the United
Kingdom
Introduction
Differences in wastewater treatment throughout Europe have led to a variety of systems being developed.
This document gives fundamental information about the systems; this document has not attempted to
specify all available systems. A generic arrangement of wastewater treatment plants is illustrated in
Figure 1:
Key:
1 preliminary treatment
2 primary treatment
3 secondary treatment
4 tertiary treatment
5 additional treatment (e.g. disinfection or removal of micropollutants)
6 sludge treatment
7 lagoons (as an alternative)
A raw wastewater
B effluent for re-use (e.g. irrigation)
C discharged effluent
D screenings and grit
E primary sludge
F secondary sludge
G tertiary sludge
H digested sludge
I digester gas
J returned water from dewatering
Figure 1 — Schematic diagram of wastewater treatment plants
The primary application is for wastewater treatment plants designed for the treatment of domestic and
municipal wastewater.
NOTE For requirements on pumping installations at wastewater treatment plants see EN 752, Drain and sewer
systems outside buildings, and EN 16932, Drain and sewer systems outside buildings — Pumping systems:
—  Part 1: General requirements;
—  Part 2: Positive pressure systems;
—  Part 3: Vacuum systems.
1 Scope
This document specifies performance requirements for treatment of wastewater using the activated
sludge process for plants over 50 PT.
The informative Annexes A to W provide design information.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
EN 16323, Glossary of wastewater engineering terms
EN 12255-1, Wastewater treatment plants - Part 1: General construction principles
EN 12255-10, Wastewater treatment plants - Part 10: Safety principles
EN 12255-11, Wastewater treatment plants - Part 11: General data required
EN 12255-12, Wastewater treatment plants - Part 12: Control and automation
3 Terms and definitions
For the purposes of this document, the terms and definitions given in EN 16323 and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
• ISO Online browsing platform: available at https://www.iso.org/obp/ui
• IEC Electropedia: available at https://www.electropedia.org/
3.1
enhanced biological phosphorus removal
activated sludge system for increased biological phosphorus removal by luxury uptake whereby mixed
liquor or return sludge is intermittently subjected to anaerobic and aerobic conditions
3.2
internal recirculation ratio
IRR
ratio of the flow of recirculated nitrate containing wastewater to a denitrification reactor relative to the
inflow
3.3
selector
first, optional reactor of an activated sludge system where incoming wastewater and return activated
sludge are blended and mixed to subject the return activated sludge to a high sludge load in order to
mitigate sludge bulking
Note 1 to entry: A selector can be aerobic or anaerobic; aerobic selectors are more common. An anaerobic selector
can also be used to assist biological phosphorus removal.
3.4
mixed liquor suspended solids
MLSS
dry mass concentration of suspended solids in a mixed liquor
[SOURCE: EN 16323:2014, definition 2.3.10.24]
Note 1 to entry: The dry mass of filtered solids is determined in accordance with the 23rd edition of Standard
Methods for Wastewater (SMEWW), 2540 parts D & E.
3.5
mixed liquor volatile suspended solids
MLVSS
dry mass concentration of organic suspended solids in a mixed liquor
[SOURCE: EN 16323:2014, definition 2.3.10.25]
Note 1 to entry: The dry mass of filtered solids is determined in accordance with the 23rd edition of Standard
Methods for Wastewater (SMEWW), 2540 parts D & E.
4 Symbols and abbreviations
4.1 Symbols
Symbol Definition Unit
A area m
C mass concentration mg/l
D diameter m
oxygen uptake rate per person (specific oxygen
OUR kg/(P∙d)
spec
consumption)
f utilization factor (see EN 12255-1) (dimensionless)
U
load (food to mass ratio), (e.g. kg (BOD /d) per
F/M kg/(kg∙d)
kg MLSS)
HRT hydraulic retention time (= V/Q) d or h
internal recirculation ratio (for recirculation of
IRR (dimensionless)
nitrate)
L length m
aerobic sludge age = mean aerobic solids
MASRT d
retention time
MSRT sludge age = mean solids retention time d
OC oxygen (transfer) capacity kg/h
kg/(P∙ d)
OC specific oxygen consumption per person
spec
oxygen transfer efficiency at operational
OTE kg/kWh
conditions
P power W or kW
Symbol Definition Unit
total population (= population + population
PT P
equivalents)
Q flow m /h or l/s
Q specific flow per person m /(P∙h)
spec
return sludge ratio = return sludge flow to
RSR (dimensionless)
wastewater inflow
standard oxygen transfer rate in clean test
SOTR kg/h
water
specific standard oxygen transfer rate in clean
SSOTR g/(Nm ∙h)
test water per standard volume of air
standard oxygen transfer efficiency in clean test
SOTE kg/kWh
water
specific standard oxygen transfer efficiency in
SSOTE clean test water (percent of supplied oxygen %/m
transferred per immersion depth)
SSP surplus sludge production kg/d
SSP specific surplus sludge production per person g/(P∙d)
spec
SVI sludge volume index ml/g
SSVI stirred sludge volume index ml/g
T temperature °C or K
V volume m
W width m
Y yield (generated biomass per mass of substrate) kg/kg
a number of scraper arms -
S
−1
b degradation rate d
c molar concentration mol/m
f factor (dimensionless)
h height or depth m
l load per person and day g/(P∙d)
m mass g or kg
n number of scraper arms or diffusers —
p pressure Pa, hPa or kPa
q specific flow relative to x m /(h∙x)
t time d, h or s
v velocity m/s
Symbol Definition Unit
alpha factor = ratio of oxygen transfer
α (dimensionless)
coefficients in wastewater to clean test water
β salinity factor of clean test water (dimensionless)
Δp pressure loss Pa or kPa
4.2 Indices (not included in the symbols or abbreviations below)
3+
Al3 trivalent aluminium (Al )
aer aeration
alk alkalinity
atm atmospheric (ambient)
B bottom
BioP enhanced biological P removal
Bl blower
BM biomass
Cla clarifier
cy cycle
Deg degraded or degradable
del delay (times for raising and lowering a scraper blade)
Den denitrification
des design
Dif diffuser
dis dissolved
Dos dosing
DS dried solids
eff effective
Fe iron
2+
Fe2 bivalent iron (Fe )
3+
Fe3 trivalent iron (Fe )
geo geodetic (vertical level)
h hourly
im immersion
in incoming
inert not degradable
inorg inorganic
int intermittent aeration
intD intermittent denitrification
kLA oxygen transfer coefficient
max maximal
min minimal
Nitr nitrification
org organic
out outgoing
part particulate
PL Pipeline
PostD post-denitrification
PreD pre-denitrification
prec precipitated
Proc process
R reactor
redeg readily degradable
ret returned
S scraper
Sal salinity
Sat saturation
SC shortcut
Scr scraper
SE scraper effectiveness
SimD simultaneous denitrification
Spec specific (related to x)
St standard
TW test water
4.3 Abbreviations
Al aluminium
BOD biochemical oxygen demand in 5 days
C carbon
CH methane
CO carbon dioxide
COD chemical oxygen demand
DS dried solids
EPDM ethylene-propylene-dien class M, a synthetic rubber material
Fe iron
H S hydrogen sulfide
MAP magnesium ammonium phosphate (struvite)
ML mixed liquor
MLSS mixed liquor suspended solids
MLVSS mixed liquor volatile suspended solids
N nitrogen
NH ammonium
NO nitrate
N O nitrous oxide (laughing gas)
orgN organic nitrogen
O oxygen
P phosphorus
PE-HD polyethylene with high density
PP polypropylene
PT total population
PVC polyvinylchloride
RS return sludge
SBR sequencing batch reactor
TKN total Kjeldahl nitrogen
TSS total suspended solids
WWTP wastewater treatment plant
5 Requirements
5.1 General
Biological reactors and final clarifiers are connected by return sludge recirculation lines and form a unit
process: the activated sludge process. The performance of the process depends on biological and
chemical reactions in the activated sludge tanks as well as separation of activated sludge in the final
clarifiers. Activated sludge systems include structures, such as aeration basins and sedimentation tanks,
and technical equipment, such as aeration systems and sludge scrapers.
Biological treatment and clarification (decanting) may be combined in a single sequencing batch reactor
(SBR) with intermittent aeration and sedimentation.
The design shall take account of the requirements specified in EN 12255-1, EN 12255-10, EN 12255-11
and EN 12255-12.
Annexes A, B and C provide typical design values, typical wastewater characteristics and usual primary
settling tanks’ effectiveness.
5.2 Planning
5.2.1 Basic information
The design of an activated sludge system may be based on common values as provided in Annex B, in
particular for plants serving up to 1 000 PT. For larger plants, the design should be based on the following
information (ideally maximum or minimum 2 weeks average over 2 to 3 years):
1. Maximum and minimum wastewater temperature and temperature-dependent requirements on the
effluent quality;
2. Maximum, minimum hourly flow and yearly average wastewater inflow; and the maximum 2 h-
inflow during dry weather conditions;
3. System loads, depending on primary treatment (where provided), including variations of COD (or
BOD5), TSS, P and TKN concentrations. The 85 %-quantiles should be provided for system design and
the 50 %-quantiles (i.e. medians) or arithmetic averages should be provided for the calculation of
operating costs and the design of sludge treatment facilities;
4. Where possible, the composition of the incoming COD shall be provided to the designer, separated
into degradable dissolved COD, inert dissolved COD, degradable particulate COD, inert particulate
COD and readily degradable COD; See Annex C for more information.
NOTE With the standard methods, COD is analysed using dichromate as the oxidising agent. Chromium is a
heavy metal. It would be more sustainable if dichromate could be replaced with a different oxidising agent.
5. A minimum of 40 samples should be analysed for all parameters. For plants serving less than
10 000 PT the number of samples may be less.
6. The consent standards concerning COD, N and P concentrations in the effluent.
Return loads from sludge treatment shall be taken into account, particularly ammonium return load. In
some cases, it may be necessary to provide separate treatment of filtrate or centrifugate from sludge
dewatering, e.g. using a de-ammonification process.
Load removal ratios during primary treatment shall be taken into account. It is recommended to
investigate the removal ratios during dry weather conditions. Where this is not feasible, removal ratios
as shown in Annex C may be used.
Biological treatment units should be protected from excessive hydraulic loads e.g. by the use of overflow
devices and/or storm tanks to meet the required discharge consent. The frequency and volume of
wastewater discharges should be limited (see EN 752).
If the waste water composition is unusual, it is recommended that a half-technical pilot test is performed
for a minimum period of half a year (including the cold weather period) to investigate data for the system
design. A design based on long-term testing can optimize the design and avoid safety factors necessarily
included in a more general design.
Where the required sample analysis is not feasible, Annex A provides basic guidance information for
system design.
The following factors shall be determined during planning of an activated sludge system:
• capacity and dimensions of the biological reactors;
• prevention of dead zones and of detrimental deposition in tanks/channels;
• establishment of multiple lines/units or other technical means to maintain the required final effluent
quality while maintenance or repair work is carried out;
• aeration and/or mixing equipment in the biological reactors with sufficient capacity;
• surface area, volume and depth of final clarifiers;
• sludge removal system within clarifiers;
• sludge recirculation and surplus sludge wasting equipment;
• internal recirculation ratio and equipment;
• sufficient stabilization of the removed surplus sludge (where required);
• measurement and control systems;
• odour control;
• noise and vibration control;
• hydraulic head loss.
It may be necessary to add easily degradable organic carbon compounds (e.g. methanol) in order to
achieve sufficient denitrification. Annex D provides information of such additives.
5.2.2 System selection
The configuration, number, shape and volume of reactors achieving the main biological reactions can vary
considerably according to:
• plant size;
• the quality of treatment to be achieved, e.g. only BOD (or carbon) removal, nitrification,
denitrification and/or phosphorus removal;
• the requirement for simultaneous aerobic sludge stabilization (i.e. the required aerobic sludge age);
• selection of a single-stage or multi-stage system;
• where biological nitrogen removal is required: selection of the type of denitrification (e.g. pre-,
cascade-, simultaneous, alternating, intermittent or post-denitrification);
• provision of anaerobic or aerobic selectors to mitigate sludge bulking;
• provision of anaerobic reactors to achieve enhanced biological phosphorus removal;
• provision of reactors which can use anoxic or aerobic treatment (depending on load and
temperature);
• requirement for chemical phosphate removal by addition of metal salts (e.g. of ferric, ferrous or
aluminium salts);
• minimum and maximum temperatures, and temperature dependent requirements (e.g. N-removal
requirements).
Where biological nitrogen removal is required, nitrification and denitrification reactors shall be provided.
Six systems can be distinguished (see Figures 2 to 7):
1. pre-denitrification in one or several anoxic reactors which are (usually) not aerated;
2. cascade denitrification with alternating anoxic and aerobic reactors whereby the inflow is fed to
anoxic reactors;
3. simultaneous denitrification in a loop reactor (oxidation ditch) with alternating aerobic and anoxic
zones;
4. alternating denitrification with parallel reactors that are sequentially aerated and non-aerated,
whereby the inflow is always fed into the non-aerated reactor;
5. intermittent aeration providing for a sequence of aerobic and anoxic conditions within a reactor, e.g.
in an SBR-reactor; intermittent aeration requires a substantially higher capacity of the aeration
system;
6. post-denitrification with a carbon source fed into the anoxic reactor, followed by subsequent
aeration (this system may be used where the C/N-ratio in the influent is so low that a carbon source
shall be added anyway).
Pre-denitrification and cascade denitrification require recycling of nitrate containing wastewater from
nitrification to denitrification reactors or zones. The internal recirculation ratio depends on the required
denitrification ratio.
Enhanced biological phosphorus removal may be provided. It may offer the following advantages:
• saving of precipitants;
• reduced dry mass of surplus sludge;
• improved possibility of phosphorus recycling;
• lower reduction of the wastewater’s alkalinity depending on the precipitant used;
• lower concentration of anions (e.g. chloride) in the effluent.
Favourable conditions for enhanced biological P-reduction are:
• high ratio of readily degradable COD to the P-content in the influent;
• low oxygen and nitrate concentration in the flows entering the anaerobic reactor;
• if the flow pattern of the anaerobic reactor is close to a plug flow reactor or where it is a cascade
reactor.
Disadvantages of enhanced Bio-P removal at plants with anaerobic sludge digestion are:
• Sometimes severe precipitation of struvite (MAP = magnesium-ammonium-phosphate) in anaerobic
digesters and related equipment;
• Dissolved phosphate binds water, reduces the effectiveness of flocculants and impairs the dewatering
results.
The addition of precipitants for P-removal is usually required. For this reason, the capability to add
precipitant dosing facilities shall always be provided for even where they are not initially included.
Selection and design of the activated sludge system may be done with the help of dynamic modelling. This
can be particularly helpful for the upgrading of existing systems.
Figure 2 to Figure 7 show process options for nitrogen removal (nitrification plus denitrification)
systems (Source: DWA-A 131).
Key
1 denitrification
2 nitrification
3 return sludge
4 internal recirculation (NO )
Figure 2 — Pre-denitrification

Key
1 denitrification
2 nitrification
3 return sludge
4 internal recirculation (NO )
Figure 3 — Cascade-denitrification

Key
1 denitrification
2 nitrification
3 return sludge
Figure 4 — Simultaneous denitrification
Key
1 denitrification
2 nitrification
3 return sludge
Figure 5 — Alternating denitrification

Key
1 / 2 denitrification or nitrification
3 return sludge
Figure 6 — Intermittent denitrification

Key
1 nitrification
2 denitrification
3 return sludge
4 post-aeration
5 organic carbon
Figure 7 — Post-Denitrification
Annex E provides information about the required sludge age depending on the requirements, Annex F
about the surplus sludge production, Annex G about the required denitrification capacity, and Annex H
about the oxygen consumption.
Annex I provides guidance how to determine the ratio of anoxic zones and Annex J about the reactor
volumes.
Annex K shows how the internal recirculation ratio for N-removal is calculated.
Annex L shows how the required alkalinity is determined.
Annex M provides guidance on the design of aerobic selectors.
In Annex N the design with F/M-ratios is explained.
Annex O provides information on the sludge volume index.
Annex P explains how the concentration of the return sludge is calculated and Annex Q how the return
sludge flow and its mixed liquor concentration are calculated.
5.2.3 Biological reactors
The process design shall be based on one of the following design parameters dependent on the required
wastewater and sludge treatment quality:
• sludge age or
• sludge loading (F/M-ratio).
Both values depend on the concentration of the mixed liquor suspended solids (MLSS) or of the mixed
liquor volatile suspended solids (MLVSS) which depend on the sludge volume index (SVI) and the
performance of the final clarifiers.
The performances of biological reactors and final clarifiers are interdependent. For this reason, activated
sludge systems shall be designed as a complete system.
Exemplary design information can be found in the informative Annex A and in the literature (see
Bibliography).
It can be useful to provide an aerated or non-aerated selector in order to mitigate the development of
sludge bulking (resulting from growth of filamentous bacteria, such as microthrix or nocardia).
Where intermittent pumping is provided, the influent and return sludge shall arrive at a selector at the
same time.
Another or additional option to mitigate sludge bulking is the temporary use of aluminium salts instead
of ferric or ferrous salts for phosphorus precipitation.
5.2.4 Clarifiers
Final clarifiers shall be provided with equipment for scum and foam removal.
Clarifiers shall provide for:
• separation of activated sludge solids from treated waste water by sedimentation;
• storing activated sludge to prevent it from overflowing during high hydraulic load;
• gravity thickening and removal of the activated sludge in order to recirculate it to the activated sludge
reactor.
Each purpose requires a special zone in clarifiers. Information about the heights of the zones is provided
in Annex S.
Sludge removal shall be slow at the bottom of the sludge thickening zone, preventing the generation of
turbulence which could jeopardize sludge thickening.
Clarifiers can be upward flow (so called Dortmund tanks), horizontal flow or lamella separators (see
EN 12255-4). Upward flow is limited to a surface area of about 100 m .
For general construction principles, the design of scrapers and their tracks, and for their design service
life see EN 12255-1.
Sequencing batch reactor (SBR) systems do not need a subsequent clarifier and sludge return equipment
because aeration, sedimentation and decanting occur intermittently within an SBR.
5.2.5 Environmental impact
Activated sludge systems can emit odour, noise and aerosols, particularly systems with surface aerators.
Such impacts should be mitigated by structural means.
Even more problematic could be the emission of greenhouse gases. Activated sludge systems require
much energy (50 % to 80 % of the entire WWTP). They oxidize carbon matter to carbon dioxide which is
released into the atmosphere. They also emit small quantities of the very strong greenhouse gases nitrous
oxide (N O) and methane (CH ).
2 4
Environmental effects should be considered before selecting a system.
5.3 Detailed Design
5.3.1 Flow-splitting
When the process involves multiple lines or parallel units, the incoming flow shall be distributed by
adjustable distribution devices (e.g. weirs, gates, or valves) that can also be used to isolate each treatment
unit.
Accumulation and removal of floating matter shall be considered during planning of flow-splitting
devices.
5.3.2 Biological reactors
Biological reactors can be completely mixed. Continuous flow reactors can also be designed to achieve
sequential reaction characteristics (e.g. close to plug flow). This can either be achieved by a series of
several mixed reactors (cascades) or by providing long reactors with a length to width ratio of about
15 : 1.
Reactors provided with fine bubble diffusion should have a minimum depth of 4 m. If such reactors are
more than 6 m deep, means for removing gas from mixed liquor should be provided between the
biological reactors and final clarifiers.
A minimum of three reactor zones shall be provided for systems with pre-denitrification, whereof the
second can be operated anoxic or aerobic, depending on temperature and load. This requires installation
of both aeration and mixing equipment.
At least two parallel reactors or subsequent cascades should be provided for all WWTPs serving more
than 10 000 PT. Each reactor shall be provided with isolation means. A bypass shall be provided for
smaller plants.
Where SBR reactors are used, at least two SBR reactors should be provided for plants serving more than
1 000 PT to equalize flow and aeration if they are not continuously fed.
If the plant is designed for one or more reactors to be taken out of service for routine maintenance, the
reactors remaining in operation and their associated pipework, channels, etc., shall have the capacity to
accommodate the design wastewater flow and ensure the required effluent quality.
NOTE Local or national regulations can permit this to be temporarily less stringent than in normal operation
in certain conditions.
Tanks shall be designed to allow emptying either by gravity flow or by pumping. Structures shall be
designed such that emptying will not affect their stability, irrespective of the groundwater level. All
necessary measures shall be taken, such as ballast concrete or facilitating lowering of the groundwater
level, in order to prevent flotation.
The floors of tanks should slope towards a low point to facilitate emptying. When a pump is used for
emptying, a drain pit should be provided at the low point.
The hydraulic design shall minimize short-circuiting. Completely mixed reactors are also possible. It is
desirable for the flow through reactors to be close to a plug flow. This can be achieved in rectangular
tanks with a meandering flow pattern or a partial blocking of the longitudinal flow.
In the case of a multipoint feed system (e.g. step-aeration), appropriate devices (e.g. weirs, gates or
valves) shall be provided to allow modification of the original flow-splitting arrangement. The same
applies to systems where the return sludge is fed at various points.
The water level in biological reactors can be controlled by fixed or adjustable overflow weirs.
The freeboard of aeration tanks shall be sufficient to prevent overflowing of mixed liquor or scum
(or foam) under normal operational conditions. A freeboard of 0,5 m should be provided.
Foam of varying stability and viscosity can develop, particularly where filamentous bacteria are
prevalent. The number of possible points of accumulation shall be minimized. In addition, bottom
openings shall be provided in walls separating compartments in reactors to prevent high water pressure
on the separating walls when the reactor is emptied.
All emissions from reactors shall comply with national requirements.
Where reactors are covered (e.g. for environmental reasons), the materials used shall be capable of
withstanding the aggressiveness of the atmosphere which shall be especially taken into account where
septic wastewater (H S-corrosion) or aggressive industrial effluents can arrive. In such cases, the walls
above the water level shall also be protected down to 0,3 m below the lowest operating water level. It
shall also be considered that ammonia can be corrosive to stainless steel. Means for explosion prevention
should be considered.
In such cases, forced ventilation can be used to limit the aggressiveness of the atmosphere and increase
the service life of structures and equipment. Forced ventilation shall be installed, if staff need to enter the
enclosed space.
5.3.3 Mixing
The mixed liquor in all reactors shall be agitated to prevent activated sludge from settling or forming
detrimental deposits. The design of tanks, piping and mixers should avoid short-circuiting of flows that
need to be mixed throughout the tank volume, e.g. incoming wastewater, returned activated sludge and
mixed liquor recirculation. Mixing can be performed with:
• aeration systems or equipment (e.g. fine-bubble diffuser systems or surface aerators);
• coarse bubble diffusion for the generation of a rotational flow patterns (with little oxygen transfer);
• mechanical mixing (e.g. with propeller or jet mixers).
A combination of several systems is possible.
Intermittent or variable speed operation of mixers can save energy.
Mechanical mixers shall be designed to minimize debris accumulation and cording by fibrous materials.
Mechanical mixers shall be removable without emptying the tank.
All mixing systems shall be designed such that they are capable of preventing the generation of solid
deposits and that they are capable to re-suspend settled solids. Where no primary clarifiers are provided,
the design of mixing systems depends on the effectiveness of preceding grit chambers (see EN 12255-3).
The choice of mixing system depends on the characteristics of the wastewater to be treated, the geometric
reactor configuration and potential short-circuiting. The electrical power consumption of mechanical
3 3 3
mixers is typically between 1 W/m and 5 W/m . A power consumption as low as 0,3 W/m may be
sufficient if the wastewater pre-treatment, geometry of the tank and the design of the mixer are
optimized.
The power consumption of mechanical mixers is a poor indicator of mixing performance. The momentum
generated by the mixer, in combination with the installation geometry, is a better criterion for the mixing
result. For axial flow submersible mixers, momentum generation is given by the mixer thrust according
to ISO 21630:2007. For long shafted axial mixers the same principle can be applied.
The momentum required to drive the mixing flow can be generated by large and slow propellers more
energy efficiently than by small and fast propellers. Propeller design and motor efficiency at the operating
points have an additional impact on energy efficiency. Large propeller mixers usually require strong
fixation because they produce strong thrust and torque. Large propellers can be more sensitive to
upstream turbulence.
Propeller mixers subject to fluctuating thrust or torque could lose their intended performance and suffer
fatigue. Such fluctuations occur if air bubbles or skewed water currents enter the propeller suction zone.
To avoid this, sufficient clearance between propeller and aeration equipment in all directions shall be
ensured by the designer or by the system supplier (if the latter supplies the aeration equipment and the
mixers). The designer shall avoid placing the propeller in strong or skewed flow, e.g. caused by pipe or
weir inlets, baffles or guide vanes, or other mixers.
Slow running propellers should be preferred to prevent destruction of flocs.
The power input of fine or coarse bubble diffusion systems, calculated as isothermal decompression
power shall be minimum 2 W/m :
PV/ = Q ⋅ p ⋅ ln p / p ≥ 2 W / m   [W/m ] (1)
( )
11 1 2
where
P is power released during decompression in W;
V is tank volume in m ;
Q is air flow at p in m /s;
1 1
p is pressure at the immersion depth in Pa;
p is atmospheric air pressure in Pa.
This requirement is usually met when the air flow of fine or coarse bubble diffuser systems is minimum
3 2
1 Nm /h per m surface area. The power consumption of the blower is far higher than the decompression
power P.
Installation of coarse bubble diffuser pipes is a simple method for mixing denitrification reactors.
Intermittent mixing can save energy.
More powerful mixing with about double the energy input, is
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